1. Field of the Invention
This invention relates to nonvolatile memory having a rewrite disabling function.
2. Description of Related Art
Nonvolatile memory in which stored content can be erased and rewritten, and in which stored content does not disappear even if the power source is cut is used for storing data which are updated frequently. It is also used for storing programs and constants. Particularly in small-scale systems, one of the storage regions in nonvolatile memory is often used both for storing data and also for storing programs and constants. In such a case, the storage region for programs and constants is set in a rewrite disabled state in order to prevent the accidental erasure or rewriting of data, programs, or constants stored in this region.
Conventionally, a rewrite disabling circuit used in nonvolatile memory is comprised of a circuit for judging whether an inputted address is a rewrite disabled region, and a circuit for halting the output of signals or the like necessary for erasing or writing when the inputted address is a rewrite disabled region, and thereby performs a storage region rewrite disabling function.
However, in conventional rewrite disabling circuits used in nonvolatile memory, the rewrite disabled region must be limited in advance, or a rewrite disabled region must be set for each constant block. As a result, it has been impossible to set a desired region as a rewrite disabled region at any time and in accordance with the conditions of use of the nonvolatile memory.
The present invention solves the aforementioned problems in the prior art by providing nonvolatile memory in which a rewrite disabled region can be set freely at any time.